Analyses of Echo-traces of Individual Fish Obtained in situ by Dual-beam Method.

Eight echo-traces of single fish were analyzed to derive information on the fish. The data-base for these analyses was collected in situ using a dual-beam system, whose operating frequency was 38kHz, on board an anchored vessel. The swimming depth and the speed of the fish were estimated to be 56-97m and 0.7-1.7m/s, respectively. The evaluated swimming orientation ranged from 5 degrees downward to 10 degrees upward. The extracted maximum target strength was transformed to the fork length for each fish, using an empirical formula. Except for one trace, the estimated lengths were distributed in a range from 45.3 to 71.1cm. The lengths were in good agreement with the length-range of walleye pollock (Theragra chalcogramma) hooked at the anchored position. The maximum target strengths were observed in the aspect of only several degrees head-down. The measurements of tethered specimens also show that the maximum values were found in a similar aspect. In one exceptional case, however, the estimated fork length (104.3cm) was far longer than the maximum size of this species (70cm), and the maximum target strength was observed at the head-down aspect of 10 degrees. Therefore, this extraordinary fish may be a different species.